1. Field of the Invention
The present invention relates to a charging device configured to be used in an image forming apparatus based on an electrophotographic technology, a process cartridge including the charging device, and an image forming apparatus including the charging device.
2. Description of the Related Art
Various types of conventional electrophotographic methods are disclosed in, for example, U.S. Pat. No. 2,297,691, Japanese Examined Patent Publication No. S49-23910, and Japanese Examined Patent Publication No. S43-24748. In the electrophotographic method, a copy is obtained by forming an electric latent image on a photoconductor by using a photoconductive material in various ways, developing the latent image with a dry toner, transferring the toner image onto a paper or the like, and fixing the toner image by applying heat and pressure.
Methods for developing the electric latent image can be roughly divided into liquid developing methods and dry developing methods. In the liquid developing methods, a developer is used, which is obtained by having various types of pigments or dyes finely dispersed in an electrically-insulating organic liquid. In the dry developing methods such as a cascade method, a magnetic brush method, and a powder cloud method, a toner is used, which is obtained by having a colorant such as carbon black dispersed in a natural resin or a synthetic resin. The dry developing methods can further be divided into one-component developing methods and two-component developing methods that require a carrier.
With increasing demand for high quality images, particles of the toner have become smaller and sphere, recently. Especially, copy images or printed images with a high definition and a high resolution have been strongly demanded. To obtain such images having the high definition and the high resolution, Japanese Patent Application Laid-Open Nos. H1-112253, H2-284158, and H7-295283 disclose a developer having a specific content and a specific distribution of toner particles each having an average diameter equal to or smaller than 5 micrometers. The toner particles having the diameter equal to or smaller than 5 micrometers are a requisite component for forming the images with the high definition and the high resolution. By smoothly supplying the toner when a latent image is developed, it is possible to obtain the image accurately reproducing the latent image without spreading out of the edges, that is, the image with a high reproductivity. However, because toner particles have become smaller and sphere, there tends to increase an amount of residual toner that still remains on the photoconductor after a cleaning device cleans the toner from the photoconductor after the image is transferred.
Japanese Patent Application Laid-open No. S63-149668 discloses a charging roller method in which a charging device is in contact with a photosensitive member during charging. Japanese Patent Application Laid-open Nos. H7-140762, H7-140868, and H2-301777 disclose methods for cleaning the charging roller.
However, in the arrangements described above, even if the amount of the residual toner that still remains on the photoconductor after the cleaning device cleans the photoconductor is extremely small, the toner gradually accumulates on the surface of the charging roller, which causes lowering in a charging efficiency over the course of time. This is not a problem to be considered in low-speed printers or copying machines that are in popular lines of products, because the charging device or the process cartridges that include a charging device are replaced at about tens of thousands printing operations. However, in medium-speed or high-speed printers or copying machines, because the replacement cycle is hundreds of thousands of printing operations, the charging efficiency is lowered due to the imperfect cleaning, and the output images have abnormalities such as background smudges over the course of time. A solution for the above problems has been in high demand.